Typical signs of apple scab can be found on apple leaves late in the season. Symptomatic spots are found on both leaves and fruit, and spores will overwinter on fallen leaves.
Photos by Sally Colby.
By the end of harvest, most growers are relieved to be finished with orchard work for a short break until it's time to start pruning. But according to Dr. Kari Peter, plant pathologist at the Penn State Fruit Research and Extension Center (FREC) in Biglerville, Pa., late fall is the ideal time to clean up the orchard - a step that can pay big dividends when it comes to managing disease.
"Sanitation practices are the best way to manage disease for the following year," she said. "Everything overwinters - fungal spores, bacteria - and anything that wasn't killed or controlled by fungicides is potentially problematic for the following year."
Peter said that removing dead and decaying wood and limbs that have cankers due to disease plays a critical role in orchard sanitation because that's where spores and bacteria overwinter. "One thing I really stress to growers is that fungi and bacteria need organic tissue to survive the winter. If organic tissue isn't there, spores can't survive," she said.
Apple scab is one disease that can be managed through late-season practices. Apple scab is caused by the ascomycete Venturia inaequalis, and it is prevalent in all apple-growing regions. By eliminating the source of scab - fallen leaves - growers can potentially reduce fungicide use and subsequent resistance issues. Flail or mulch mowing in fall or winter will help to chop fallen leaves so that naturally occurring microbial organisms in the ground can aid in further breakdown. Flail mowing also moves leaves around so spores aren't facing up toward the tree.
Another option for apple scab management is fall application of urea. "It's usually applied when the tree is dormant so that excess nitrogen won't stimulate growth and damage the tree," Peter explained. "Urea can be sprayed when leaves are on the tree and still falling, or when leaves have fallen on the ground." Growers who apply urea in the fall should be prepared to adjust springtime fertilizer applications accordingly.
Pruning will help control many diseases, including fire blight, which overwinters in cankers on limbs from the previous season. "Fire blight is typically an issue in spring when there are blossoms," said Peter, noting that fire blight occurred this past summer at FREC. "The bacteria were present in spring, but didn't have the environmental cues because we had a cool, slow spring." Peter explained that bacteria move when shoots are growing, and that growth makes shoots susceptible. She advises growers who see evidence of fire blight midseason not to take action until the tree stops growing, because that's when the disease stops spreading. Dormant copper is the recommended treatment for fire blight, and it will also help control apple scab the following season.
After flail or mulch mowing to break up fallen leaves and fruit, mulched material will break down with the help of soil microbes.
By eliminating dead wood that can harbor fungus, diseases such as summer rot can be controlled. Peter said that any dead wood that has cankers or signs of disease should be removed from the orchard and burned. "You don't want it around for the next season," she said. "Orchards are self-infecting - spores don't go very far. If you remove the inoculant source, you're minimizing the chance for scab the following year."
Peter added that younger trees present more of a challenge when it comes to controlling disease through pruning, simply because there's less to prune.
Diseases such as fire blight can potentially move faster in young trees, so it's important for growers to recognize and deal with it early. "If growers haven't had the opportunity to deal with fire blight as soon as they see the early symptoms, it has a greater opportunity to get into the vascular system of the plant and move around," she said. "You lose the opportunity to get ahead of the disease and may have problems the following year."
Another important disease control measure is eliminating dead hanging fruit from trees. Peter said that mummified fruit doesn't fall on its own and can serve as an inoculant source the following year. The mummified fruit will often be removed during pruning, but in some cases the tree would have to be overpruned to remove all of it. Growers who aren't applying urea have to be even more mindful when it comes to eliminating fallen fruit. "Some of those apples will develop rot that produces fungal spores," said Peter. "Wind or splashing rain moves spores up into the bark, and those spores overwinter in the bark."
Apples with scab may remain on trees into late fall and should be removed to minimize the chance of spores overwintering on the orchard floor.
At the end of the season, growers should seek out wild relatives of fruit trees that potentially harbor disease. Because growers don't typically manage diseases in wild species, there's potential for diseases in wild trees to spread to orchards. One example is black knot in plum trees, which can originate from wild plum and cherry trees growing in nearby hedgerows.
Peter reminds growers to read fungicide and pesticide labels carefully, and to be aware of which products can be used on each species.
The author is a frequent contributor and freelance writer who farms and raises Great Pyrenees in south-central Pennsylvania. Comment or question? Visit http://www.farmingforumsite.com and join in the discussions.